This invention relates to thermal storage and more particularly is an improved method and apparatus for utilizing ice which is formed and stored in a vessel.
Thermal storage equipment of the type which forms ice during off peak energy periods and then makes the ice available as a supply of cold for space conditioning, and the like, is known. In one form of such thermal storage equipment a refrigerant liquid, such as brine or an ethylene glycol solution, is flowed through a channel which is immersed in a pool of freezeable storage liquid, such as water. The pool of water, or the like, is confined within a vessel and the refrigerant channel usually is a form of tubing bent into a serpentine with plural tube runs immersed in the pool. Plural refrigerant channels are usually packed in parallel within the pool and connected between inlet and outlet headers which receive and discharge the refrigerant liquid from, and to, one or more heat exchangers in which the refrigerant liquid is cooled during the ice production cycle, and warmed during the cold supply cycle. The storage liquid is usually agitated during at least certain periods of operation to lessen temperature stratification.
During the ice production cycle cold refrigerant liquid, at a temperature below the solidification point of the storage liquid within the pool, is continuously produced by mechanical refrigeration, or the like, in one heat exchanger (usually referred to as a "chiller") and flowed to the inlet header through the channels and out of the outlet header and returned to the chiller. The storage liquid will freeze on the channels in the form of surrounding envelopes and gradually develop a substantial thickness of frozen liquid (usually ice). At a point just before the envelopes on parallel adjacent channels contact one another the optimum effective storage capacity will be reached. However, a quantity of unfrozen storage liquid will normally remain free along the walls of the vessel and between adjacent frozen envelopes, and such free liquid will equilibrate at a temperature close to the freezing point.
During the supply cycle the refrigerant liquid is circulated to a heat exchanger (such as a component of a space air conditioning system), where the refrigerant is warmed, and returned to the channels within the thermal storage unit where it is cooled by the frozen envelopes. However, as each envelope melts internally to form a liquid sleeve around the refrigerant channel, the liquid sleeve will increase in temperature above the storage liquid freezing point and to an extent will partially insulate the channel surface from the remaining frozen envelope. This results in a temporary increase in the temperature of the refrigerant liquid exiting from the outlet header (above the storage liquid freezing point) thereby lowering the design parameter of the thermal storage unit due to its inability to continuously deliver refrigerant liquid close to the freezing temperature of the storage liquid. The latter condition, although temporary, will continue until the frozen envelope is opened by heat convection of the liquid sleeve whereupon the agitated free liquid, exterior of the envelope, also becomes available to chill the tube surface. The condition may also be partially relieved at such time that the buoyant effect of the free liquid in the vessel lifts the frozen envelope sufficiently to urge the ice against the lower portions of tube surface.
Thus it would be highly advantageous to overcome the effect of the liquid sleeve that forms between tube and frozen envelope during an early part of the supply cycle.